Hello Friends,
               Google moved from Google Cloud Messaging (GCM) to Firebase Cloud Messaging (FCM). Just like GCM, FCM is a cross-platform messaging solution that allows you to send messages. FCM is completely free and there are no limitations.

• Firebase Message types : Using FCM you can send three types of messages i.e:
• Notification Message
• Data Message
• both Notification & Data Payload

• Integrating Firebase Cloud Messaging 
• First thing you need to do is go to https://firebase.google.com/ and make an account to gain access to their console. After you gain access to the console you can start by creating your first project.
• Give the package name of your project in which you are going to integrate the Firebase. Here the google-services.json file will be downloaded when you press add app button.

• Sending Messages from Firebase Console : Go to firebase console(https://console.firebase.google.com/project/fir-notification-d7287/notification) and click on Notifications in left menu. You will get an interface where you can type the message, select the targeting and send the message.

1. MainActivity.java

package android.developer.solutions.firebasenotification.activity;

import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.content.SharedPreferences;
import android.developer.solutions.firebasenotification.R;
import android.developer.solutions.firebasenotification.app.Config;
import android.developer.solutions.firebasenotification.util.NotificationUtils;
import android.os.Bundle;
import android.support.v4.content.LocalBroadcastManager;
import android.support.v7.app.AppCompatActivity;
import android.text.TextUtils;
import android.util.Log;
import android.widget.TextView;
import android.widget.Toast;

import com.google.firebase.messaging.FirebaseMessaging;

public class MainActivity extends AppCompatActivity {

    private static final String TAG = MainActivity.class.getSimpleName();
    private BroadcastReceiver mRegistrationBroadcastReceiver;
    private TextView txtRegId, txtMessage;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        txtRegId = (TextView) findViewById(R.id.txt_reg_id);
        txtMessage = (TextView) findViewById(R.id.txt_push_message);

        mRegistrationBroadcastReceiver = new BroadcastReceiver() {
            @Override
            public void onReceive(Context context, Intent intent) {

                // checking for type intent filter
                if (intent.getAction().equals(Config.REGISTRATION_COMPLETE)) {
                    // gcm successfully registered
                    // now subscribe to global` topic to receive app wide notifications
                    FirebaseMessaging.getInstance().subscribeToTopic(Config.TOPIC_GLOBAL);

                    displayFirebaseRegId();

                } else if (intent.getAction().equals(Config.PUSH_NOTIFICATION)) {
                    // new push notification is received

                    String message = intent.getStringExtra("message");
                    Toast.makeText(getApplicationContext(), "Push notification: " + message, Toast.LENGTH_LONG).show();
                    txtMessage.setText(message);
                }
            }
        };

        displayFirebaseRegId();
    }

    // Fetches reg id from shared preferences
    // and displays on the screen
    private void displayFirebaseRegId() {
        SharedPreferences pref = getApplicationContext().getSharedPreferences(Config.SHARED_PREF, 0);
        String regId = pref.getString("regId", null);

        Log.e(TAG, "Firebase reg id: " + regId);

        if (!TextUtils.isEmpty(regId))
            txtRegId.setText("Firebase Reg Id: " + regId);
        else
            txtRegId.setText("Firebase Reg Id is not received yet!");
    }

    @Override
    protected void onResume() {
        super.onResume();

        // register GCM registration complete receiver
//        LocalBroadcastManager.getInstance(this).registerReceiver(mRegistrationBroadcastReceiver,
//                new IntentFilter(Config.REGISTRATION_COMPLETE));

        // register new push message receiver
        // by doing this, the activity will be notified each time a new message arrives
        LocalBroadcastManager.getInstance(this).registerReceiver(mRegistrationBroadcastReceiver,
                new IntentFilter(Config.PUSH_NOTIFICATION));

        // clear the notification area when the app is opened
        NotificationUtils.clearNotifications(getApplicationContext());
    }

    @Override
    protected void onPause() {
        LocalBroadcastManager.getInstance(this).unregisterReceiver(mRegistrationBroadcastReceiver);
        super.onPause();
    }
}

2. MyFirebaseInstanceIDService.java

package android.developer.solutions.firebasenotification.service;

import android.content.Intent;
import android.content.SharedPreferences;
import android.developer.solutions.firebasenotification.app.Config;
import android.support.v4.content.LocalBroadcastManager;
import android.util.Log;

import com.google.firebase.iid.FirebaseInstanceId;
import com.google.firebase.iid.FirebaseInstanceIdService;


/**
 * Created by Mukesh on 10/02/17.
 * www.androiddevelopersolutions.com/
 */
public class MyFirebaseInstanceIDService extends FirebaseInstanceIdService {
    private static final String TAG = MyFirebaseInstanceIDService.class.getSimpleName();

    @Override
    public void onTokenRefresh() {
        super.onTokenRefresh();
        String refreshedToken = FirebaseInstanceId.getInstance().getToken();

        // Saving reg id to shared preferences
        storeRegIdInPref(refreshedToken);

        // sending reg id to your server
        sendRegistrationToServer(refreshedToken);

        // Notify UI that registration has completed, so the progress indicator can be hidden.
        Intent registrationComplete = new Intent(Config.REGISTRATION_COMPLETE);
        registrationComplete.putExtra("token", refreshedToken);
        LocalBroadcastManager.getInstance(this).sendBroadcast(registrationComplete);
    }

    private void sendRegistrationToServer(final String token) {
        // sending gcm token to server
        Log.e(TAG, "sendRegistrationToServer: " + token);
    }

    private void storeRegIdInPref(String token) {
        SharedPreferences pref = getApplicationContext().getSharedPreferences(Config.SHARED_PREF, 0);
        SharedPreferences.Editor editor = pref.edit();
        editor.putString("regId", token);
        editor.commit();
    }
}

3. MyFirebaseMessagingService.java

package android.developer.solutions.firebasenotification.service;

import android.content.Context;
import android.content.Intent;
import android.developer.solutions.firebasenotification.activity.MainActivity;
import android.developer.solutions.firebasenotification.app.Config;
import android.developer.solutions.firebasenotification.util.NotificationUtils;
import android.support.v4.content.LocalBroadcastManager;
import android.text.TextUtils;
import android.util.Log;

import com.google.firebase.messaging.FirebaseMessagingService;
import com.google.firebase.messaging.RemoteMessage;

import org.json.JSONException;
import org.json.JSONObject;


/**
 * Created by Mukesh on 10/02/17.
 * www.androiddevelopersolutions.com/
 */
public class MyFirebaseMessagingService extends FirebaseMessagingService {

    private static final String TAG = MyFirebaseMessagingService.class.getSimpleName();

    private NotificationUtils notificationUtils;

    @Override
    public void onMessageReceived(RemoteMessage remoteMessage) {
        Log.e(TAG, "From: " + remoteMessage.getFrom());

        if (remoteMessage == null)
            return;

        // Check if message contains a notification payload.
        if (remoteMessage.getNotification() != null) {
            Log.e(TAG, "Notification Body: " + remoteMessage.getNotification().getBody());
            handleNotification(remoteMessage.getNotification().getBody());
        }

        // Check if message contains a data payload.
        if (remoteMessage.getData().size() > 0) {
            Log.e(TAG, "Data Payload: " + remoteMessage.getData().toString());

            try {
                JSONObject json = new JSONObject(remoteMessage.getData().toString());
                handleDataMessage(json);
            } catch (Exception e) {
                Log.e(TAG, "Exception: " + e.getMessage());
            }
        }
    }

    private void handleNotification(String message) {
        if (!NotificationUtils.isAppIsInBackground(getApplicationContext())) {
            // app is in foreground, broadcast the push message
            Intent pushNotification = new Intent(Config.PUSH_NOTIFICATION);
            pushNotification.putExtra("message", message);
            LocalBroadcastManager.getInstance(this).sendBroadcast(pushNotification);

            // play notification sound
            NotificationUtils notificationUtils = new NotificationUtils(getApplicationContext());
            notificationUtils.playNotificationSound();
        } else {
            // If the app is in background, firebase itself handles the notification
        }
    }

    private void handleDataMessage(JSONObject json) {
        Log.e(TAG, "push json: " + json.toString());

        try {
            JSONObject data = json.getJSONObject("data");

            String title = data.getString("title");
            String message = data.getString("message");
            boolean isBackground = data.getBoolean("is_background");
            String imageUrl = data.getString("image");
            String timestamp = data.getString("timestamp");
            JSONObject payload = data.getJSONObject("payload");

            Log.e(TAG, "title: " + title);
            Log.e(TAG, "message: " + message);
            Log.e(TAG, "isBackground: " + isBackground);
            Log.e(TAG, "payload: " + payload.toString());
            Log.e(TAG, "imageUrl: " + imageUrl);
            Log.e(TAG, "timestamp: " + timestamp);


            if (!NotificationUtils.isAppIsInBackground(getApplicationContext())) {
                // app is in foreground, broadcast the push message
                Intent pushNotification = new Intent(Config.PUSH_NOTIFICATION);
                pushNotification.putExtra("message", message);
                LocalBroadcastManager.getInstance(this).sendBroadcast(pushNotification);

                // play notification sound
                NotificationUtils notificationUtils = new NotificationUtils(getApplicationContext());
                notificationUtils.playNotificationSound();
            } else {
                // app is in background, show the notification in notification tray
                Intent resultIntent = new Intent(getApplicationContext(), MainActivity.class);
                resultIntent.putExtra("message", message);

                // check for image attachment
                if (TextUtils.isEmpty(imageUrl)) {
                    showNotificationMessage(getApplicationContext(), title, message, timestamp, resultIntent);
                } else {
                    // image is present, show notification with image
                    showNotificationMessageWithBigImage(getApplicationContext(), title, message, timestamp, resultIntent, imageUrl);
                }
            }
        } catch (JSONException e) {
            Log.e(TAG, "Json Exception: " + e.getMessage());
        } catch (Exception e) {
            Log.e(TAG, "Exception: " + e.getMessage());
        }
    }

    /**
     * Showing notification with text only
     */
    private void showNotificationMessage(Context context, String title, String message, String timeStamp, Intent intent) {
        notificationUtils = new NotificationUtils(context);
        intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
        notificationUtils.showNotificationMessage(title, message, timeStamp, intent);
    }

    /**
     * Showing notification with text and image
     */
    private void showNotificationMessageWithBigImage(Context context, String title, String message, String timeStamp, Intent intent, String imageUrl) {
        notificationUtils = new NotificationUtils(context);
        intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_CLEAR_TASK);
        notificationUtils.showNotificationMessage(title, message, timeStamp, intent, imageUrl);
    }
}

4. AndroidManifest.xml

Download code from here

Hope you like this...
Enjoy Coding..... :)